docker: Upgrade to Ubuntu 20.04

Ubuntu 20.04 comes with CMake 3.16, so there's no need to install
a custom version of CMake for now (for LLVM 16.x; LLVM 17 will have
the requirement raised to CMake 3.20 though).

Also drop the workaround for git issues in the old version in
Ubuntu 18.04.

The latest releases that are built on GitHub Actions without
docker are based on Ubuntu 20.04 anyway, so there's less demand
for building this with an older version for the sake of
compatibility of the built binaries. For users using the built
docker image as such, having it based on a newer base distribution
is beneficial.

release.sh

@@ -29,7 +29,7 @@ fi
 
 time docker build -f Dockerfile . -t mstorsjo/llvm-mingw:latest -t mstorsjo/llvm-mingw:$TAG
 
-DISTRO=ubuntu-18.04-$(uname -m)
+DISTRO=ubuntu-20.04-$(uname -m)
 docker run --rm mstorsjo/llvm-mingw:latest sh -c "cd /opt && mv llvm-mingw llvm-mingw-$TAG-ucrt-$DISTRO && tar -Jcvf - llvm-mingw-$TAG-ucrt-$DISTRO" > llvm-mingw-$TAG-ucrt-$DISTRO.tar.xz
 
 if [ -n "$NATIVEONLY" ]; then